This disclosure relates to an improved audio signal processing system for use in audio amplification and recording systems, and in the voice controlled activation of radio transmitters, recording devices, and other ancillary equipment.
Since the early days of electronic audio amplification, methods have been devised to automatically control the gain of electronic amplifiers so that a wide range of input levels can be accommodated while maintaining a fixed output level. This technology is useful in Public Address applications to prevent speaker overload and excess amplification of the audio input, and in the recording industry to prevent overdriving the recording equipment. Over the years, various modifications of this technology have been introduced, all of which use some form of negative feedback to alter performance characteristics.
In addition to audio compression and level control, voice activation techniques have emerged whereby the presence of an audio signal is detected and a control signal generated so that ancillary equipment, such as recording devices or radio transmitters, can be activated.
As a variation of voice activation, the presence of an audio signal has been used to activate a switch in the audio signal path in a manner that interrupts the signal path when the audio input levels fall below a given threshold. This "Voice Gate" technology is frequently used to eliminate background noise and undesired breath sounds between words, and to mute unused inputs in a multiple channel audio circuit.
All of this technology suffers because the processes are applied in real time on the incoming audio signal. Since a finite time is required to analyze the signal and calculate the appropriate response parameters, transient control is inaccurate. In the case of speech compression, the initial amplification will be inappropriately high, distorting the output signal. Likewise, activation of the voice gate and ancillary voice operated equipment is delayed, so that the initial audio information is improperly processed or lost altogether.